Between 100,000 to 300,000 Americans die annually from pulmonary embolism (PE)—more than breast cancer and AIDS combined—representing the 3rd leading cause of death in the US [1-5]. A similar incidence of PE is found in Europe with approximately 370,000 annual deaths [6]. Moreover, PE is the 3rd most common cause of death in trauma patients that survive the first 24 hours. An estimated 25% of all hospitalized patients have some form of deep vein thrombosis (DVT) which is often clinically unapparent unless PE develops [7]. On average, 33% of DVT will progress to symptomatic PE of which 10% will be fatal [6].
The US Surgeon General has recognized this alarming statistic and in 2008 issued a formal Call to Action to Prevent DVT and PE [1]. Unfortunately, DVT/PE disproportionately affects the elderly, in part due to prolonged periods of inactivity following medical treatment. The incidence is relatively low under the age of 50 (1/100,000), then accelerates exponentially reaching 1000/100,000 by the age of 85 [8]. Consequently the US Surgeon General has proclaimed that the growth in number of DVT/PE cases with an aging US population may outpace the population growth in the absence of better prevention [1].
Risk factors for PE arising from DVT follow Virchow's Triad [9]: (i) endothelial injury, (ii) hypercoaguability, and (iii) hemodynamic changes (stasis or turbulence). Hence specific risk factors include hip and knee arthroplasty, abdominal, pelvic and extremity surgeries, pelvic and long bone fractures, prolonged immobility such as prolonged hospital stays and air travel, paralysis, advanced age, prior DVT, cancer, obesity, COPD, diabetes and CHF. Orthopedic surgeons are especially concerned since their patients carry a 40%-80% risk for DVT and PE following knee and hip surgeries in the absence of prophylactic treatment [10-12].
The American Academy of Orthopaedic Surgeons (AAOS) has issued guidelines for PE prophylaxis. Basically, patients at standard risk should be considered for chemoprophylactic agents such as aspirin, low molecular weight heparin (LMWH), synthetic pentassaccharides, or warfarin, in addition to intra-operative and/or immediate postoperative mechanical prophylaxis [13].
Aspirin has a 29% relative risk reduction in symptomatic DVT and a 58% relative risk reduction in fatal PE [14]. LMWH carries a 30% risk reduction in DVT and has been proven more effective than unfractionated heparin in high risk groups such as hip and knee arthroplasty [7]. Warfarin started within 24 to 48 hours of initiating heparin with a goal of achieving international normalized ratio (INR) results between 2 and 3 as secondary thromboprophylaxis for 3 months reduces the risk of recurrent venous thromboembolism (VTE) by 90% as compared with placebo [15,16]. Mechanical prophylaxis, consisting of pneumatic compression devices that repeatedly compress the legs with an air bladder, are also utilized in conjunction with anticoagulants to reduce the occurrence of PE.
The duration of prophylaxis depends on the source of potential DVT. Current recommendations for prophylaxis consist of a minimum 7-10 days for moderate to high risk surgeries and up to 28-35 days for many orthopedic surgeries. Specifically for orthopedic trauma, DVT prophylaxis is continued until patient mobilization (32%), inpatient discharge (19%), 3 weeks postop (16%), 6 weeks postop (27%), and in rare circumstances greater than 6 weeks (7%) [17]. Studies indicate that hypercoaguability persists for at least one month after injury in 80% of trauma patients [18]. Regarding total knee and hip arthroplasty and cancer surgeries, 35 day prophylactic treatment is recommended [12, 19]. Overall, prophylactic treatment for possible VTE is often warranted for up to 6 weeks following trauma or major surgery.
Contraindications for chemoprophylaxis include active bleeding, hemorrhagic diathesis, hemorrhagic stroke, neurologic surgery, excessive trauma, hemothorax, pelvic or lower extremity fractures with intracranial bleeding, anticoagulation interruption, and recent DVT/PE patients undergoing surgery.
For patients who are contraindicated for the above-mentioned anti-coagulation prophylaxis, or where anti-coagulation therapy has failed, the AAOS, American College of Physicians, and the British Committee of Standards in Haematology all recommend the use of inferior vena cava (IVC) filters [13, 20, 21]. These intravascular metal filters are deployed via catheter into the IVC to essentially catch emboli arising from DVT before reaching the lungs resulting in PE. Furthermore, the British Committee of Standards in Hematology recommends IVC filter placement in pregnant patients who have contraindications to anticoagulation and develop extensive VTE shortly before delivery (within 2 weeks).
The Eastern Association for Surgery of Trauma further recommends prophylactic IVC filters placed in trauma patients who are at increased risk of bleeding and prolonged immobilization [22]. Such prophylactic recommendation follows studies that demonstrate a low rate of PE in patients with severe polytrauma who underwent IVC placement [23-25]. In fact the fastest growing indication of overall IVC filter usage, from 49,000 in 1999 to 167,000 in 2007 with a projected 259,000 units for 2012, is the prophylactic market utilizing retrievable IVC filters [26, 27].
Example vascular filters primarily for IVC placement are disclosed in U.S. Pat. Nos. 4,425,908; 4,655,771, 4,817,600; 5,626,605; 6,146,404; 6,217,600 B1; 6,258,026 B1; 6,497,709 B1; 6,506,205 B2; 6,517,559 B1; 6,620,183 B2; U.S. Pat. App. Pub. No. 2003/0176888; U.S. Pat. App. Pub. No. 2004/0193209; U.S. Pat. App. Pub. No. 2005/0267512; U.S. Pat. App. Pub. No. 2005/0267515; U.S. Pat. App. Pub. No. 2006/0206138 A1; U.S. Pat. App. Pub. No. 2007/0112372 A1; U.S. Pat. App. Pub. No. 2008/0027481 A1; U.S. Pat. App. Pub. No. 2009/0192543 A1; U.S. Pat. App. Pub. No. 2009/0299403 A1; U.S. Pat. App. Pub. No. 2010/0016881 A1; U.S. Pat. App. Pub. No. 2010/0042135 A1; and U.S. Pat. App. Pub. No. 2010/0174310 A1.
IVC filter efficacy has been demonstrated in several class I and II evidence studies [22, 28-30]. Most of the earlier filters installed were expected to be permanent fixtures since endothelialization occurs within 7-10 days making most models impractical to remove without irreversible vascular damage leading to life threatening bleeding, dissection of the IVC, and thrombosis. Although these permanent filters have prevented PE, they have been shown to actually increase the risk of recurrent DVT over time.
Specifically, a Cochrane review [31] on the use of IVC filters for the prevention of PE cites a level I randomized prospective clinical trial by Decousus et al. [32] wherein the incidence of DVT with the IVC filter cohort increased almost 2-fold: (i) 21% incidence of recurrent DVT in the filter cohort vs. 12% in the non-filter LMWH cohort at 2 years (p=0.02), and (ii) 36% incidence of recurrent DVT in the filter cohort vs. 15% in the non-filter group at 8 years (p=0.042) [33]. However, the filters did reduce the occurrence of PE; the filter cohort experiencing only 1% PE vs. the non-filter cohort posting 5% PE in the first 12 days (p=0.03). No statistically significant difference in mortality rate was seen in any time frame investigated. Apparently the initial benefit of reduced PE with permanent IVC filters is offset by an increase in DVT, without any difference in mortality.
In addition to increased incidence of DVT for prolonged IVC filter deployment, filter occlusion has been reported with a 6% to 30% occurrence, as well as filter migration (3% to 69%), venous insufficiency (5% to 59%), and post thrombotic syndrome (13% to 41%) [34-36]. Complications from insertion including hematoma, infection, pneumothorax, vocal cord paralysis, stroke, air embolism, misplacement, tilting arteriovenous fistula, and inadvertent carotid artery puncture have an occurrence rate of 4%-11% [37].
Temporary or retrievable IVC filters have been marketed more recently intended to be removed once the risk of PE subsides, and hence circumvent many of the deleterious complications of permanent filters. The retrievable filters feature flexible hooks, collapsing components, and unrestrained legs to ease retrieval. Unfortunately these same features have led to unwanted filter migration, fatigue failure, IVC penetration, fragment migration to hepatic veins and pulmonary arteries, filter tilt, and metallic emboli [38-43]. Since 2005, 921 adverse filter events have been reported to the FDA including 328 device migrations, 146 device detachments (metallic emboli), 70 perforations of the IVC, and 56 filter fractures [44]. Some retrievable brands post alarming failure rates such as the Bard Recovery filter with 25% fracturing over 50 months which embolized end organs. 71% of the fractures embolized to the heart caused life threatening ventricular tachycardia, tamponade, and sudden death in some cases. An alternative retrievable model, Bard G2, resulted in 12% fractures over 24 months [45]. Such prevalence of device fractures is postulated to be directionally proportional to indwell time.
These failures and others prompted the FDA in August 2010 to issue a formal communication stating that “FDA recommends that implanting physicians and clinicians responsible for the ongoing care of patients with retrievable IVC filters consider removing the filter as soon as protection from PE is not longer needed” [44]. Even though these types of retrievable filters are intended to be removed in months time, several studies indicate that approximately 70%-81% of patients with retrievable IVC filters fail to return to the hospital for filter removal, thereby exposing hundreds of thousands of patients to the life-threatening adverse events of prolonged retrievable IVC filter placement [41, 44, 46-48]. These patients are either lost to follow-up, or refuse to have the filters removed in the absence of complications.